1. Field of the Invention
The present invention relates to a method and apparatus for the removal of volatile contaminants in soil. Specifically, the present invention relates to a method and apparatus for performing high speed, high capacity soil remediation at a site where the soil is contaminated with volatile compounds. Most particularly, the present invention relates to a method and apparatus for achieving thermal remediation of various soil mixtures contaminated with volatile hydrocarbon compounds.
2. Description of the Related Art
Hydrocarbon contamination of soils, gravel and sand is a worldwide environmental problem. Because of the potential for evaporation of the contaminants into the air and leaching into the local ground water supply, safe, efficient and complete removal techniques of hydrocarbons are needed.
The problem of decontamination of sites where hydrocarbons such as gasoline, jet fuels, and oils have been spilled and absorbed into the ground is complicated by not only the variety of hydrocarbon contaminants possible, but the characteristics of the surrounding environment where cleanup is desirable or necessary. Soil compositions are typically multi-phase and generally include nitrogen and oxygen, water, clays, dirt, rocks, and sand (i.e., small rocks and silicon dioxide grains). Therefore, distribution of the volatile organic compounds and other contaminants in gravel, sand or soil, and mixtures thereof, herein referred to collectively as "soil", depends on the varying volumetric composition of the soil. Consequently, within a given lot of multi-phase soil, the contaminants are distributed according to the permeability and uniformity of the soil and the chemical and physical relationship between the contaminants and the soil as described by the appropriate distribution coefficients.
One means for decontaminating or remediating soil containing hydrocarbon or other contaminants is by combustion of the contaminants by exposure to a high temperature environment, processes generally referred to as thermal remediation. Such a high temperature environment, ranging from as low as about 500.degree. F. to 2000.degree. F. (260.degree. C. to 1093.degree. C.) more, may be created in a variety of ways including direct exposure of the contaminated soil to an infrared radiation source or, more simply, a flame. Contaminated soil may also be heated indirectly, for example, by a heat exchange relation where the soil is isolated from a hot oil or hot gas.
Heating the contaminated soil to high temperatures tends to insure that all hydrocarbon contaminants contained therein are combusted, thereby decontaminating the soil. Heating the contaminated soil to relatively low temperatures, e.g., about 400.degree. F. (204.degree. C.) or less, may require multiple recirculations of the soil to prolong exposure to the heated environment and maximize the amount of hydrocarbons combusted.
While higher temperatures can facilitate rapid and thorough combustion, enough heat can be generated thereby to raise the temperature of the soil to 600.degree. F. (315.degree. C.) or more. In addition, exhaust gases and entrained fine particulate species on "fines" produced as a result of combustion will be very hot. Generally, the hot gases and fines must be separated from the soil after the combustion steps since the soil has thereby been decontaminated and may be suitable, after cooling, for return to the worksite from which it was excavated.
Typical separation means known in the art include inertial separators, such as cyclone separators or baghouses. Ceramic filters are also known. Alternatively, fines entrained in the exhaust gases from the combustion step may be removed by using a scrubber or stripping column filled with a liquid or a gas to capture the particulate species. Columns filled with carbon may be used to adsorb fines based on organic residues. Such processes inherently introduce problems of proper selection and disposal of the liquid solvent or the carbon adsorbent containing the captured fines. In particular, the solvent or adsorbent used may depend on the particulate species involved. Finally, a gas use to strip the exhaust gases may require additional stripping treatment in order to remove the particulate species before the stripping gas itself may be disposed.
In addition, high temperatures of the gases and fines exhausted from the combustion step will impact the choice for the separation operation. For example, the exhaust gases and fines may be so hot that either conventional filters used in baghouse-type separators are burned or the liquid solvent may vaporize, precluding separation altogether.
As discussed above, a principal concern for remediation processes is the need for a high throughput capacity in order to quickly and efficiently treat the huge quantities of soil which are likely to be involved in a contaminated worksite before the soil is suitable for return to the environment. Some systems currently available for thermally remediating are described as being capable of processing up to about 120 tons per hour. But, high throughput is not merely a question of scaling up the components used in a remediation apparatus, since substantial reduction of the temperatures of the soil, gases and fines may be necessary before any can be reintroduced to the environment. Indeed, achievement of high throughput results from optimization of heat input to the soil, heat transfer from the soil, gases and fines, and equipment input and output capacities in light of the characteristics of the contaminated soil to be treated.